This invention relates generally to the treatment of nitrogen-rich natural gas streams. In particular, the invention relates to the recovery of helium and natural gas liquid (NGL) and optionally methane from pressurized natural gas comprising predominantly nitrogen with smaller amounts of methane, C2+ hydrocarbons and helium.
U.S. Pat. No. 3,531,943 discloses a cryogenic process for separation of a natural gas with a high nitrogen content. The reference exemplifies (see figure) a process for treating a natural gas containing about 60 vol. % nitrogen and about 39 vol. % hydrocarbons (mostly methane), together with small amounts of carbon dioxide, helium, argon and hydrogen sulfide. The gas is fed at 2000 psi (138 bar) to a purification unit using monoethanolamine to remove carbon dioxide and hydrogen sulfide. The gas is then dried and cooled to about 49° F. (about 9° C.). Any condensed hydrocarbons are separated and the gas is then further cooled to −96° F. (about −71° C.). Any more condensed hydrocarbons are separated and the gas is then expanded to form a two phase stream (15 mol. % liquids) at 400 psi (about 28 bar) and −197° F. (−127° C.). The gas/liquid mixture is then further cooled and partially condensed (36 mol. % liquids) and fed at −203° F. (−131° C.) to a tower where it is separated into nitrogen overhead and methane bottoms. It is disclosed that helium may be recovered by adding a staged condenser or dephlegmator above the overhead condenser in the tower.
U.S. Pat. No. 5,167,125A discloses a process for recovering light gases such as hydrogen, neon and helium, from gas stream containing higher boiling components such as nitrogen and C1-2 hydrocarbons. According to the embodiment depicted in FIG. 1 of U.S. Pat. No. 5,167,125A, a stream 100 of feed gas is cooled by indirect heat exchange and the cooled feed gas 110 is reduced in pressure across valve 112 and fed to a distillation column 102 where it is separated into bottoms liquid depleted in light gas(es), and overhead vapor enriched in light gas(es). The bottoms liquid is reboiled using the feed gas in reboiler 108 to provide vapor for the column. Nitrogen in the overhead vapor is condensed in the overhead condenser 116 by indirect heat exchange against a stream 104 of bottoms liquid that is expanded across valve 122, and the resultant liquid nitrogen is recycled to the column as reflux 120. A stream 118 of impure helium gas is removed from condenser 16.